The present invention relates to a solid state image sensor.
As known, the solid state image sensor is so arranged as to read out signal charges, which are stored in individual photosensitive cells formed on a semiconductor substrate, following the photoelectric conversion of light, by some means through an output section formed in the semiconductor substrate. In the case of the image pick-up tube, a target film for performing the photoelectric conversion and the storage of the signal charge is formed of a single film. The read out of the signal charge stored in the target film is performed by the scanning electron beam. Since a sheet resistance of a continuous target film is very high, little signal charge is laterally diffused, so that the resolution is almost determined by a diameter of the scanning beam.
In the solid state image sensor, it is impossible to read out the signal charge by the scanning beam, unlike the image pick-up tube. For this reason, the resolution in the image pick-up tube is determined by a density of the photosensitive regions and the signal read out out regions packed in the image sensor.
In one of the known solid state image sensors, the photoelectric conversion is performed in the semiconductor substrate and then the signal charge is stored in the sensor. With this structure, attainment of a given resolution requires a large number of the photosensitive region and provision of an overflow drain region in the substrate in order to prevent an excessive signal charge caused by the photoelectric conversion performed in the substrate from overflowing into the read out region. In this respect, the prior image sensor has a limit in improving the integration density.
To cope with this problem, there has recently been proposed another solid state image sensor with a structure that the photoelectric conversion is executed by the photoelectric converting film and the signal charge generated by the photoelectric converting film is read out by a read out region formed in the substrate. The image sensor will be described referring to FIG. 1. As shown, N.sup.+ conductivity regions (first regions) 2.sub.1, 2.sub.2, . . . electrically connected to conductor electrodes 10.sub.1, 10.sub.2, . . . , are laterally formed, at fixed intervals in a substrate of the P conductivity type, for example. These N.sup.+ conductivity regions 2.sub.1, 2.sub.2, . . . are used for storing the signal charge produced through the photoelectric conversion by a photoelectric converting film 11. The substrate 1 has also N.sup.+ conductivity regions 3.sub.1, 3.sub.2, . . . for reading out the signal charge from the N.sup.+ regions 2.sub.1, 2.sub.2, . . . , which are disposed adjacent to the N.sup.+ storage regions 2.sub.1, 2.sub.2, . . . with gate regions 4.sub.1, 4.sub.2, . . . each with a given length interposed therebetween. Stopper regions 5.sub.1, 5.sub.2, . . . , formed in the substrate 1, separate one another unit blocks each including a combination of one of these storage regions 2.sub.1, 2.sub.2, . . . and one of these read out regions 3.sub.1, 3.sub.2, . . . Polycrystal silicon gate electrodes 7.sub.1, 7.sub.2, . . . as transfer electrodes are formed over gate insulation films 6.sub.1, 6.sub.2, . . . in the regions of the substrate 1 where the gate regions, the N.sup.+ read out regions, and the stopper regions are located. An insulation film 8 made of, for example, is layered over the substrate 1 containing the gate electrodes 7.sub.1, 7.sub.2, . . . Contact holes 9.sub.1, 9.sub.2, . . . allowing the conductor electrodes 10.sub.1, 10.sub.2, . . . to electrically contact with the corresponding storage regions 2.sub.1 , 2.sub.2, . . . are formed corresponding to the N.sup.+ storage regions 2.sub.1, 2.sub.2, . . . A plurality of the conductor electrodes 10.sub.1, 10.sub.2, . . . formed over the insulation film 8 are disposed separately at given distance. The conductor electrodes 10.sub.1, 10.sub.2, . . . are electrically connected to N.sup.+ storage regions 2.sub.1, 2.sub.2, . . . , respectively. The photoelectric converting film 11 made of amorphous silicon, for example, for executing the photoelectric conversion, is formed over the entire surface of the insulation film 8. A predetermined voltage is applied to a transparent electrode 12 layered over the photoelectric converting film 11.
In operation, light rays are irradiated over the region of the photoelectric converting film 11 over the conductor electrode 10.sub.1, under a condition that the predetermined voltage is applied to the transparent electrode 12. The photoelectric converting film 11 is activiated to generate signal charge. The signal charge generated is transferred through the conductor electrode 10.sub.1 and stored in the N.sup.+ storage region 2.sub.1. For reading out the signal charge stored, a voltage is applied to the gate electrode 7.sub.2. Upon the application of the voltage, the signal charge is transferred through the gate region 4.sub.1 to the N.sup.+ read out region 3.sub.1.
With such a structure of the image sensor, the photoelectric converting film 11 formed over the substrate 1 executes the photoelectric conversion. This feature provides the more improvement of the integration density in the image sensor without reducing the resolution than the image sensor of the type in which the photoelectric conversion is performed in the photosensitive region formed in the substrate. In the image sensor as described referring to FIG. 1, when an excessive amount of the signal charge is generated at the time of the photoelectric conversion, the signal charge can be discharged to exterior through the transparent electrode 12 on the photoelectric converting film 11. The result is that there is no need of the overflow drains adjacent to the photosensitive regions in the substrate which would otherwise be required like the image sensor of the in-substrate photoelectric conversion type. This feature also contributes to the improvement of the integration density.
In the image sensor as mentioned above, the N.sup.+ storage region 2.sub.1, the N.sup.+ read out region 3.sub.1, the gate electrode 7.sub.2, the conductor electrode 10.sub.1 and the photoelectric converting film 11 make up one cell, or the unit block. A plurality of the cells are arranged in a matrix fashion, as shown in FIG. 2. In FIG. 2, some components have different suffixes although having the same reference numerals as those in FIG. 1, for simplicity of illustration. This is correspondingly applied to the related drawings to be referred to in the specification. In the figure, reference symbols 3a to 3d designate N.sup.+ regions as the read out regions, and 9a to 9p contact portions for providing electrical contact of the storage regions with the conductor electrodes 10a to 10p. In the image sensor with a matrix pattern of the conductor electrodes 10a to 10p, the light rays incident on the photoelectric converting film (FIG. 1) on the conductor electrodes 10a to 10 p are subjected to the photoelectric conversion. The signal charge resulting from the photoelectric conversion process is transferred to the N.sup.+ read out regions 3a to 3d located under the conductor electrodes 10a to 10p, as indicated by arrows. Picture elements, which will frequently be used in the description to follow, corresponds to a single conductor electrode.
In the NTSC standard television system, the image sensor has a matrix of the conductor electrodes of 500 in the vertical or column direction and 400 in the horizontal or row direction. The image sensor thus constructed has no particular problem in the vertical direction, but has a problem in the horizontal direction. In the horizontal direction, the spatial frequency 400 c/s on the black and white bar pattern reachs the Nyquist limit frequency. When a pattern more detailed than the Nyquist limit frequency is picked up, a false signal called Moire appears to remarkably damage the reproduced picture on the screen.
Particularly, in the solid state image sensor, the false signal is large. The reason for this is that in this type of the image sensor, the conductor electrodes defining the picture elements are formed independently one another, and, as a result, the output signal and a degree of amplification when the bar pattern at the Nyquist limit frequency is picked up are large. Such phenomenon can be alleviated by merely increasing the number of the picture elements or the conductor electrodes in the horizontal direction. This approach, however, encounters a technical difficulty in the manufacturing stage of the sensor. In addition to this, it suffers from increase of the signal read rate, resulting in difficulty of the circuit design of the drive circuit and the signal processing circuit and the like.
There is another proposal in the solid state image sensor of the in-substrate photoelectric conversion type. In the proposal, a plurality of the conductor electrodes along one row line in the matrix are disposed shifted displaced by 1/2 picture elements with respect to those on the preceeding and succeeding row lines in the horizontal or row direction. That is, the picture elements are disposed in a zig-zag fashion as viewed in the vertical or column direction.
When two or three image sensors with the structure as mentioned above are combined in a manner that the effective region of one image sensor overlaps with the ineffective region of other sensors the resolution is improved. In the case of one chip image sensor of the zig-zag type, it is difficult to fabricate the sensor of a high integration density. Additionally, in the image sensor of this type, the intensitive region to the light increases, causing the false signal and thus little improving the integration density. To improve the resolution, a plurality of chips must be used so as to obtain the signal charges from the entire area receiving the light rays. This measure essentially leads to increase of the manufacturing cost, and a complicated circuit design of the drive circuit and the signal processing circuit.